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Parabolic dishes are favoured because they are "perfect". Unfortunately they are only perfect if they are kept constantly pointed at the sun. In practice this is very hard to do. A better shape would maintain good reflection of the sunlight for a long time without moving the dish.I think I have found useful easy software to help with this design work. It is called art of illusion and you can see my first results and a downloadable scene file to help you get started on step 12 of this instructable https://www.instructables.com/id/Make_a_quotcompoundquot_parabolic_solar_cooker/Brian White